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Setback savings - fact or fiction?

 
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Air Source Heat Pumps (ASHPs)
Last Post by JamesPa 2 hours ago
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 JamesPa
(@jamespa)
Illustrious Member Moderator
Joined: 3 years ago
Posts: 3941
12/01/2026 9:55 pm  

Posted by: @sunandair
↑

@jamespaamespa, thank you both @iaack so far I haven’t calculated this latent heat of fusion. Just the raising of the 180L of water temperature from 25C to 40C. It doesn’t seem insignificant. But please let me know if my maths is wrong.

Maths looks right to me, my only question about this calculation is whether its reasonable to assume that the whole 180l is reduced in temperature to 25C.  The heat pump measures the temperature at the heat pump, ie just after its been cooled, I would guess that much of the water in the system might be warmer, possibly quite a lot warmer.  A bit of maths on pump speed etc might help give an angle on this question but it might also be necessary to observe flow temp and state of defrost (or feel the radiators!).  The min flow temp may not correspond with the end of defrost in which case the amount of water at a higher temp than the min could be even more than a calaculation based on pump speed might indicate.

My point above is, however, that the latent heat of fusion is already contained within the amount needed to reheat the water, because thats where the energy corresponding to the latent heat of fusion has come from.  So to add it in would be double counting this amount.  Its interesting to calculate it nevertheless to compare with the reheat energy, if only to allow us to ask the question - where does the rest go?  This will also serve as a sense check and conservation of energy tends to do.

Depending on exactly what you are trying to work out so might adding in "the electrical energy to replace the 12 minutes when the HP is not adding heat energy to the system during the defrost."

I think it might be easier (or perhaps a complementary sense check) to do an integration on power consumption during the defrost 'spike' which occurs immediately after defrost.  I posted this diagram from mine (7kW Vaillant) earlier

image

 

An 'eyeball integration' of the spike (ignoring the dip) suggests about 0.25kWh.  This will be largely the reheat energy I imagine.  However that doesn't account for the fact that the consumption in the period after the spike may be higher than it would have been in the absence of defrost, but of course that is partially replacing the energy that was not put in during the dip in the cases where there is one.  If you complete your calculation perhaps we can compare two ways of getting at the same question which may give us a view of the uncertainty!

Thats probably served to confuse rather than clarify in which case I apologise.

 


This post was modified 2 hours ago 4 times by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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